Nitrofurantoin is an antibacterial agent with numerous reported occurrences of pulmonary injury in animals and man. Previous studies indicated that nitrofurantoin is enzymatically reduced in lung microsomal preparations to an unstable reactive anion which has been implicated as a possible cause of the nitrofurantoin induced lung toxicity in vivo. The anion radical is thought to reduce oxygen to superoxide which can subsequently react with cellular constituents. The present study was undertaken to examine if intact lung tissue could reductively metabolize nitrofurantoin in a similar manner as pulmonary microsomes and if the resulting metabolism was modified by oxygen. Experiments were performed using a recirculating in situ perfused rat lung procedure. A specific and sensitive high pressure liquid chromatographic assay was established to analyze the resulting nitrofurantoin metabolites. Parallel studies were undertaken in lung homogenates in order to compare the metabolism of nitrofurantoin in homogenates to that obtained in intact lung tissue. Fixed perfused lung tissue has also been examined by light and electron microscopy in order to evaluate the effects of the experimental procedure on the integrity of the lung. At least two metabolites of nitrofurantoin were evident in perfused lungs after 1 hr exposure. Further, considerable binding to tissue macromolecules was also measured. Both metabolism and covalent binding were inhibited by oxygen. Similar results were observed in rat lung homogenates. Experiments are presently underway to identify the nitrofurantoin-derived metabolic products.